The present invention relates to thermal barrier coatings having a relatively thin thermally glazed surface layer for protection and mitigation against environmental contaminants, in particular oxides of calcium, magnesium, aluminum, silicon, and mixtures thereof that can become deposited onto such coatings. The present invention further relates to articles with thermal barrier coatings having such glazed surface layers and a method for preparing such coatings for the article.
Thermal barrier coatings are an important element in current and future gas turbine engine designs, as well as other articles that are expected to operate at or be exposed to high temperatures, and thus cause the thermal barrier coating to be subjected to high surface temperatures. Examples of turbine engine parts and components for which such thermal barrier coatings are desirable include turbine blades and vanes, turbine shrouds, buckets, nozzles, combustion liners and deflectors, and the like. These thermal barrier coatings are deposited onto a metal substrate (or more typically onto a bond coat layer on the metal substrate for better adherence) from which the part or component is formed to reduce heat flow and to limit the operating temperature these parts and components are subjected to. This metal substrate typically comprises a metal alloy such as a nickel, cobalt, and/or iron based alloy (e.g., a high temperature superalloy).
The thermal barrier coating usually comprises a ceramic material, such as a chemically (metal oxide) stabilized zirconia. Examples of such chemically stabilized zirconias include yttria-stabilized zirconia, scandia-stabilized zirconia, calcia-stabilized zirconia, and magnesia-stabilized zirconia. The thermal barrier coating of choice is typically a yttria-stabilized zirconia ceramic coating. A representative yttria-stabilized zirconia thermal barrier coating usually comprises about 7% yttria and about 93% zirconia. The thickness of the thermal barrier coating depends upon the metal substrate part or component it is deposited on, but is usually in the range of from about 3 to about 70 mils (from about 75 to about 1795 microns) thick for high temperature gas turbine engine parts.
Under normal conditions of operation, thermal barrier coated metal substrate turbine engine parts and components can be susceptible to various types of damage, including erosion, oxidation, and attack from environmental contaminants. At the higher temperatures of engine operation, these environmental contaminants can adhere to the heated or hot thermal barrier coating surface and thus cause damage to the thermal barrier coating. For example, these environmental contaminants can form compositions that are liquid or molten at the higher temperatures that gas turbine engines operate at. These molten contaminant compositions can dissolve the thermal barrier coating, or can infiltrate its porous structure, i.e., can infiltrate the pores, channels or other cavities in the coating. Upon cooling, the infiltrated contaminants solidify and reduce the coating strain tolerance, thus initiating and propagating cracks that cause delamination, spalling and loss of the thermal barrier coating material either in whole or in part.
These pores, channel or other cavities that are infiltrated by such molten environmental contaminants can be created by environmental damage, or even the normal wear and tear that results during the operation of the engine. However, this porous structure of pores, channels or other cavities in the thermal barrier coating surface more typically is the result of the processes by which the thermal barrier coating is deposited onto the underlying bond coat layer-metal substrate. For example, thermal barrier coatings that are deposited by (air) plasma spray techniques tend to create a sponge-like porous structure of open pores in at least the surface of the coating. By contrast, thermal barrier coatings that are deposited by physical (e.g., chemical) vapor deposition techniques tend to create a porous structure comprising a series of columnar grooves, crevices or channels in at least the surface of the coating. This porous structure can be important in the ability of these thermal barrier coating to tolerate strains occurring during thermal cycling and to reduce stresses due to the differences between the coefficient of thermal expansion (CTE) of the coating and the CTE of the underlying bond coat layer/substrate.
For turbine engine parts and components having outer thermal barrier coatings with such porous surface structures, environmental contaminant compositions of particular concern are those containing oxides of calcium, magnesium, aluminum, silicon, and mixtures thereof. See, for example, U.S. Pat. No. 5,660,885 (Hasz et al), issued Aug. 26, 1997 which describes these particular types of oxide environmental contaminant compositions. These oxides combine to form contaminant compositions comprising mixed calcium-magnesium-aluminum-siliconoxide systems (Ca—Mg—Al—SiO), hereafter referred to as “CMAS.” During normal engine operations, CMAS can become deposited on the thermal barrier coating surface, and can become liquid or molten at the higher temperatures of normal engine operation. Damage to the thermal barrier coating typically occurs when the molten CMAS infiltrates the porous surface structure of the thermal barrier coating. After infiltration and upon cooling, the molten CMAS solidifies within the porous structure. This solidified CMAS causes stresses to build within the thermal barrier coating, leading to partial or complete delamination and spalling of the coating material, and thus partial or complete loss of the thermal protection provided to the underlying metal substrate of the part or component.
Accordingly, it would be desirable to protect these thermal barrier coatings having a porous surface structure against the adverse effects of such environmental contaminants when used with a metal substrate for a turbine engine part or component, or other article, operated at or exposed to high temperatures. In particular, it would be desirable to be able to protect such thermal barrier coatings from the adverse effects of deposited CMAS.